This protocol reflects our efforts toward elucidating the role of serotonin in the pathogenesis of psychiatric illness. Currently we are examining the utility of this protocol in the study of serotonin dysfunction in adult pervasive developmental disorders and in Tourette Disorder. Studies to date have indicated that serotonin function may be more important for the mechanism of action of certain psychotropic drugs (e.g., antidepressants), than in the actual pathogenesis of the illness. Future studies include clarification of the neurobiologic implications of tryptophan depletion in humans, with an examination of effects on serotonin receptor function and central neurochemistry as reflected in cerebrospinal fluid metabolite measurements.